A voltage controlled oscillator (VCO) or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. The magnitude of the output signal is dependent on the design of the VCO circuit and the frequency of operation is determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs affect the performance of PLLs. In addition, PLLs are typically considered essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are also important in wireless networks as they enable the communications equipment to quickly lock onto the carrier frequency on which communications are transmitted.
The popularity of mobile telephones has renewed interest in and generated more attention in wireless architectures. This popularity has further spawned renewed interest in the design of low noise wideband oscillators. The recent explosive growth in the new families of cellular telephones and base stations using universal mobile telephone systems (UMTS) has stirred a need for developing an ultra-low noise oscillator with a fairly wide tuning range. The demands of wideband sources have generally increased telescopically because of the explosive growth of wireless communications. In particular, modern communication systems are typically multi-band and multi-mode, therefore requiring a wideband low noise source that preferably allows simultaneous access to DCS 1800, PCS 1900 and WCDMA (wideband code division multiple access) networks by a single wideband VCO.
The dynamic operating range and noise performance of a VCO may limit or affect the performance of the PLL itself, which in turn may affect the performance of the device in which the PLL is employed, e.g., RF transceivers, a cell phone, a modem card, etc. Broadband tunability of VCOs represents one of the more fundamental tradeoffs in the design of a VCO, impacting both the technology and the topology used. The dynamic time average quality factor (i.e., Q-factor) of the resonator as well as the tuning diode noise contribution affect the noise performance of a VCO. Furthermore, the dynamic loaded Q is, in general, inversely proportional to the operating frequency range of the VCO.
Despite the continuous improvement in VCO technology, low phase noise typically remains a bottleneck and poses a challenge to RF transceiver (transmitter-receivers) design. In addition, oscillator/VCO design typically poses a challenge to the RF trans-receiver system. This is typically considered due to the more demanding parameters of the VCO design: low phase noise, low power consumption and wide frequency tuning range.
Improvements in oscillator/VCO technology have continued with time, yielding ever-smaller sources with enhanced phase noise and tuning linearity but the phenomena of the thermal drift over the temperature range (−40° C. to +85° C.) has not been properly addressed. The wide operating temperature range of the oscillator/VCOs coupled with a general lack of information on the thermal drift-profile creates a need for the development of a uniform and user-definable thermal drift profile oscillator with a relatively low thermal drift over the wide operating temperature range and operating frequency band.
Usually, high-stability oscillators are built with a quartz crystal up to frequencies of several hundred megahertz. However, in order to achieve better stability and lower costs, the SAW (surface acoustic wave) resonator based oscillator is generally considered a better choice for an ultra low phase noise low thermal drift oscillator. SAW resonators are typically used in oscillators as a two-port resonator and have a relatively small pull-in range that usually does not support a sufficient tuning range to compensate for tolerances due to the circuit components and thermal drift over the operating temperature range (−40° C. to +85° C.). In addition, SAW devices are comparatively expensive compared to CROs (ceramic resonator based oscillator) and their availability and performance are limited to a selected frequency and narrow operating temperature range (−20° C. to +70° C.) making them unsuitable for operating in stringent temperature environments and/or low cost applications.
In addition, the thermal drift of a ceramic resonator based oscillator/VCOs is typically around 5–10 MHz over a temperature range of −40° C. to +85° C. The ceramic resonator based VCO is usually also susceptible to phase hits that may occur in a PLL.
Thus, there is a need for a user-definable thermal drift oscillator operable over a wide temperature range, which offers a cost-effective solution to the phase hit problem.